SEMICONDUCTOR GAS SENSOR FOR DETECTING AIR POLLUTION

Over 50 million tin dioxide gas sensors have been used in domestic gas alarms since 1968to 1990 only in Japan. Many other oxides are gas sensitive as LICKE iron oxides,chromium oxides, zinc oxide, etc. In this paper we characterize gas sensitive sensors madein our laboratory. A hot filament heats the oxide material of the gas sensor (deposited on aceramic cylinder) to the functioning temperature (100-400°C). When it is exposed to anatmosphere containing a reducing gas (a gas which can interact with the oxygen from theair), the electric resistance of the semiconducting material is dramatically modified, even atvery low gas concentrations. We present the influence of the supplied electric current to thesensor sensitivity and the response time and recovery time. The advantages for usingsemiconducting gas sensors for detection of the air pollution (toxic and/or flammable gases)are emphasized.

INTRODUCTION 

Air pollution by noxious exhaust gases from autovehicles’ engines andfrom burning methane gas and oil is becoming a serious danger. For effective prevention of air pollution a stable and inexpensive sensor for detecting dangerousgases is needed.In the scientific literature, various oxides were proposed as gas-sensor elements, like iron oxides, chromium oxides, zinc oxide, etc. Tin dioxide (SnO2) is by far the most popular semiconductor oxide used in semiconductor gas sensors,mainly for its ability to sense hydrocarbures and carbon monoxide. The tin dioxideis used as a thin film or as a thick film sintered powder. Naoyoshi Taguchidesigned the semiconductor gas sensors based on tin dioxide in 1962 [1]. Only inJapan there have been used over 50 million Taguchi sensors in domestic gas alarmssince 1968 to 1990 [3].The main mechanism of operation is through surface electricalconductivity changes of the semiconductor induced by chemical reactions on thesurface [4]. Atmospheric oxygen is chemisorbed on the surface primarily as O– ,and it ties the electronic carriers, decreasing the electrical conductivity of the N-type semiconductor sensor. Any reducing gases that may be present in theatmosphere will remove the chemisorbed oxygen, liberating electronic carriers intothe conduction band of the semiconductor and enhancing its the electricalconductivity. Therefore any given mixture of atmospheric oxygen and a reducinggas will produce a unique sensor conductance for that gas concentration.

THE DEVICE 

As it is show in figure 1, the semiconductor sensor for gas detection madein our laboratory contains:1.a cylindrical ceramic body (Al2O3)2.a coiled filament for heating (stainless steel AISI 304, 50µm diameter),3.a pair of platinum contacts,4.the semiconductor material, SnO2, deposited over the contacts,5.a porous case for protection, made from sintered bronze balls, gas permissive,6.a mounting base which connect the sensor’s inner wires with external wires.The inner hot filament heats the semiconducting material of the gas sensor (placedon the ceramic cylinder) to the functioning temperature (100-400°C). The platinumwires make a nonrectifying contact with the semiconductor (tin dioxide).porous caseplatinum contactfilament coilterminalsmounting baseceramic tube (Al2O3)SnO2. Sensor's structure (protected by a porous bronze case and closed by a mounting base with the terminal leads): Al2O3 cylinder with inner heater filament coil and externalSnO2 semiconductor sensing material, which have two platinum contacts.

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